Alannah Hallas
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Materials innovations have sparked most of the major technological advances across the millennia – from the stone, bronze, and iron ages through to the current silicon age. Science now stands on the precipice of a new era: the age of quantum materials – materials with extraordinary electronic and magnetic properties that rely on quantum mechanical effects. In the Hallas group, we use state-of-the-art crystal growth techniques to discover new quantum materials that could unlock these future technologies.
Crystal growth of new materials
Our group uses a wide range of synthetic methods to grow samples of the materials we study. Conventional solid state methods (shake-and-bake) and flux crystal growth are ideally suited to exploratory synthesis in the pursuit of exciting new materials. The optical floating zone image furnace is a powerful tool that allows us to grow pristine large single crystals. High pressure methods allow us to capture metastable phases that cannot be grown under ambient pressure conditions, an excellent route to finding new structural phases with the potential for exotic new properties. By using this diverse set of synthetic techniques, we are able to explore the periodic table in an unconstrained way, applying the most favourable method for the material we seek to grow.
Structure and the role of disorder
Understanding the crystallography of our new material provides the foundation upon which all other characterizations rest. First and foremost, the crystal symmetry and the connectivity of our lattice informs which theoretical models may be applicable to our material. Furthermore, it is often the materials with the most interesting ground states that exhibit the most profound sensitivity to disorder. Thus, it is crucial to determine what types of disorder are present, and attempt to modify the crystal growth recipe to obtain the highest quality samples. To accomplish these structural characterizations, our starting point is always x-ray diffraction. From there on, we can expand to other tools such as neutron diffraction and electron microscopy.
Magnetic and electronic phenomena
Quantum materials can have remarkable magnetic and electronic states, ranging from superconductors to spin liquids to topological semimetals. These states often emerge under extreme conditions, very low temperatures and high magnetic fields. We have the ability to measure a wide range of physical properties, including magnetic susceptibility, heat capacity, and electrical resistivity, down to 0.05 K (1/20th of a degree above absolute zero!) and magnetic fields up to 14 T.
Seeing deeper with neutrons and muons
While we can perform many measurements in our very own lab, some experimental techniques require us to travel to beam lines at large user facilities. We are lucky that Canada's only muon source, TRIUMF, is conveniently located on UBC campus. We can use muon spin relaxation experiments to understand whether our magnetic material is frozen or dynamic or to determine the penetration depth in our superconductor. To access neutron beams we have to travel further; Canada does not currently have a major neutron source. Neutron scattering experiments can tell us the arrangement of magnetic moments in a magnetically ordered material or to map out the the spin excitations. Muon and neutron experiments provide critical insights into the behaviors of quantum materials, that in some cases cannot be accomplished with any other experimental probe.
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Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Considerably more experimental and theoretical progress has been made in studying localmoment magnets compared to itinerant magnets, systems where magnetic moment carriersare delocalized throughout the crystal lattice. Progress is hindered by the small number ofknown itinerant magnets, difficulties in synthesis, and small moment sizes. In this thesis,we study itinerant magnetism in the intermetallic compound ZrZn₂. Presumed to order ferromagnetically, recent evidence from the local probes of muon spin relaxation (μSR) andtime dependent perturbed angular correlation (TDPAC) suggests ZrZn₂’s ordered phase isa more complex magnetic ordering. We find that polycrystalline ZrZn₂ is best synthesizedthrough solid state reaction at reaction temperatures below 700 °C. Impurity phase percentages are found to increase with temperature, with ZrZn₃ being the dominant impurity above800 °C. Bulk magnetization measurements reveal ZrZn₂ is an unsaturated magnet up to 7T,with a saturated moment size 0.18 μB/f.u. We find saturated moment sizes at 2K, in externalfields, are only 10% of free moment sizes obtained from Curie-Weiss like fits to the magnetic susceptibility. Furthermore, Knight shift measurements show the zero field orderedmoment corresponding to simple ferromagnetic ordering is only 4% of the free moment.As this is suggestive of geometric moment cancellations in the ordered phase of ZrZn₂,we employ μSR to probe its magnetic ordering in zero field (ZF) and dynamical processescontributing to this ordering in longitudinal field (LF). ZF μSR spectra displays oscillationsin the ordered phase (below 15.5K) that are more damped than the only previous report inliterature, hinting at inhomogeneities in the measured sample. In the LF geometry, we findnon-zero residual relaxation rates in ZrZn₂ persist up to 2K, which is atypical of simpleferromagnets. We conclude that while the temperature dependence of LF relaxation ratesabove 15.5K and inverse magnetic susceptibility in the paramagnetic regime agrees with theself consistent renormalization theory (SCR) for itinerant ferromagnets, a large differencein ordered and free moment size and non-zero LF relaxation rates up to 2K are suggestiveof magnetic ordering more complex than simple ferromagnetism in ZrZn₂.
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High entropy oxides (HEOs) are compounds defined as having a crystallographically ordered structure with substantial configurational entropy from having up to five elements occupying a site equivalent sublattice. In certain HEOs, the disorder functions as the driving force behind the stabilization of the crystal structure. Therefore, HEOs are capable of providing host to hitherto unexplored combinations of elements, particularly rare earth and 3d/4d transition metals, regardless of atomic mass, radii, charge, and magnetic character. The applications of these materials include reversible energy storage, components for electronic devices, and improved optical coatings. In this thesis I present two of my contributions to the domain of entropy materials from their exploratory synthesis to a robust characterization of their magnetic and physical properties. The first topic is focused on the magnetic properties of spinel-type HEO (MnCrFeCoNi)₃O₄ and a novel series of magnetic dilutions through the imposition of gallium onto the magnetic sublattice. Using SQUID magnetometry and neutron diffraction we confirm the presence of long-range antiferromagnetic order persistent throughout the parent and magnetically diluted samples. Moreover, magnetization as a function of field data reveals that replacing magnetic transition metals with gallium remarkably enhances the saturation and retentivity of the bulk magnetic moment. Shifting perspectives, we also explored a novel four component medium entropy oxide, (TiHfZrSn)O₂. The purpose of this investigation was to exploit the high tolerance for a wide dispersion in atomic mass among entropy materials in order to convolute the phonon scattering and recover enhanced thermal insulating properties. After fitting our measurements of heat capacity to a Debye model, we calculate the Debye temperature TD = 415.03 +/- 0.25 K. This is considerably lower than TD for the individual oxides and motivates further study of the phonon scattering with direct methods. Through further investigation into the αPbO₂ structure of (TiHfZrSn)O₂, we suspect that entropy stabilization plays a role in the structure formation for this compound. This is based on an investigation in the literature for a similar compound, (TiZr)O₄ in αPbO₂, where the entropy stabilized structure was realized almost two decades prior to the first reported entropy stabilized HEO.
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